Java ArrayList null pointer exception

Question

EDIT

I've cleaned up the .equals method for string equality and changed the ContactsCollection initialization to:

public static ArrayList<Contact> contactList = new ArrayList<Contact>();

I've also changed the action performed method in hopes that 'Display contacts' would show more than one contact.

    if (contactInput.equals("Display contacts"))
    {
        ContactsCollection.read();
        for (int i = 0; i < contactList.size(); i++)
        {
            contact = (Contact)contactList.get(i);
            for (int j =0; j < contactList.size(); j++)
            {
                textArea.append(contact.getName() + "," + contact.getNumber() + "\n");
            }
        }
    }

Ultimately the .dat file is written but does not contain any data that is added through the GUI.

END EDIT

I am writing a mock cellphone GUI that acts as a very basic contacts manager. There are several other classes that do not deal with the ArrayList that are working as intended.

When attempting to add a contact to the file I receive a null pointer exception at line 13 of the ContactsCollection class:

for (int i = 0; i < contactList.size(); i++)

and line 93 of the Contacts (GUI) class:

contactList.add(contact);

I have a feeling that I did something wrong when coding the Contacts and ContactsCollection classes. I'm hoping the program to run as follows: The user clicks add contact and enters the information which becomes an object Contact and is added to the contactList ArrayList and written (serialized) to a file "contactList.dat". When the user clicks display contacts the file is read in and each contact is displayed in GUI.

I think that there are several issues with the way I set up the ArrayList, but I think I'm very close to having the program run as I had hoped. Any help is greatly appreciated!

Contacts class:

import java.util.*;
import java.io.*;

public class Contact implements Serializable
{
public static final long serialVersionUID = 42L;
public String name, number;

Contact()
{
    name = "No name";
    number = "No number";
}

Contact (String theName, String theNumber)
{
    this.name = theName;
    this.number = theNumber;
}

public void setName(String aName)
{
    this.name = aName;
}

public void setNumber(String aNumber)
{
    this.number =aNumber;
}

public String getName()
{
    return name;
}

public String getNumber()
{
    return number;
}

public String toString()
{
    return name + ": " + number;
}

public boolean equals(Contact other)
{
   if (name.equals(other.getName()) && number.equals(other.getNumber()))
   {
      return(true);
   }
   else
   {
      return(false);
   }
}   
}

Contacts Collection class

import java.io.*;
import java.util.*;

class ContactsCollection
{
public static ArrayList<Contact> contactList;

public static void write()
{
    try 
    {
        ObjectOutputStream out = new ObjectOutputStream(new FileOutputStream("contactList.dat"));
        for (int i = 0; i < contactList.size(); i++)
        {
            out.writeObject(contactList.get(i));
        }
        out.close();
    } 
    catch(IOException e)
    {
        e.printStackTrace();
    }
}

public static void read()
{
contactList = new ArrayList<Contact>();
try
{
    ObjectInputStream in = new ObjectInputStream(new FileInputStream("contactList.dat"));
    Contact temp;
    while (in.available()!=0)
    {
        temp = (Contact)in.readObject();
        contactList.add(temp);
    }
    in.close();
}
catch(FileNotFoundException e)
{
    e.printStackTrace();
}
catch(IOException e)
{
    e.printStackTrace();
}
catch(ClassNotFoundException e)
{
    e.printStackTrace();
}       

}
}

Contacts (GUI) class

import java.awt.*;
import javax.swing.*;
import java.awt.event.*;
import javax.swing.Timer;
import java.util.*;

class Contacts extends JFrame implements ActionListener, WindowListener
{
public static final int WIDTH = 400;
    public static final int HEIGHT = 600;
    public static final int SMALL_WIDTH = 200;
    public static final int SMALL_HEIGHT = 100;

private static final Dimension stdBtn = new Dimension(150, 50);    

    JPanel centerPanel, northPanel, southPanel;
ImageIcon icon;
JLabel picture;
JButton addContact, displayContacts;
JScrollPane scroll;
JTextArea textArea;
Clock clock;
Background background;
Contact contact;
ArrayList<Contact> contactList;


public Contacts()
{
    super("Contacts");
    this.setLayout(new BorderLayout());
    this.setSize(WIDTH, HEIGHT);
    this.setDefaultCloseOperation(JFrame.DO_NOTHING_ON_CLOSE);
    addWindowListener(this);
    this.setLocationRelativeTo(null);

    centerPanel = new JPanel();
    northPanel = new JPanel();
    southPanel = new JPanel();

    centerPanel.setBackground(Color.BLACK);
    southPanel.setBackground(Color.BLACK);      

    clock = new Clock();
    northPanel.add(clock);

    icon = new ImageIcon("ContactsBackground.jpg");
    picture = new JLabel(icon);
    centerPanel.add(picture);

    textArea = new JTextArea("", 10, 30);
    textArea.setEditable(false);
    JScrollPane scroll = new JScrollPane(textArea, JScrollPane.VERTICAL_SCROLLBAR_ALWAYS,
            JScrollPane.HORIZONTAL_SCROLLBAR_AS_NEEDED);    
    centerPanel.add(scroll);    

    JButton displayContacts = new JButton("Display contacts");
    displayContacts.addActionListener(this);
    southPanel.add(displayContacts);

    JButton addContact = new JButton("Add contact");
    addContact.addActionListener(this);
    southPanel.add(addContact);

    this.add(northPanel, BorderLayout.NORTH);
    this.add(centerPanel, BorderLayout.CENTER);
    this.add(southPanel, BorderLayout.SOUTH);       

    setResizable(false);        
}

public void actionPerformed(ActionEvent e)
{
    contactList = new ArrayList<Contact>();
    JButton source = (JButton)e.getSource();
    String contactInput = source.getText();

    if (contactInput == "Display contacts")
    {
        ContactsCollection.read();
        for (int i = 0; i < contactList.size(); i++)
        {
            contact = (Contact)contactList.get(i);
            textArea.setText(contact.getName() + "," + contact.getNumber() + "\n");
        }
    }
    if (contactInput == "Add contact")
    {
        String name = JOptionPane.showInputDialog(null, "Enter Name");
        String number = JOptionPane.showInputDialog(null, "Enter Number");
        contact = new Contact(name, number);
        contactList.add(contact);
        ContactsCollection.write();
    }
}

public void windowOpened(WindowEvent e)
{}

public void windowClosing(WindowEvent e)
{
    this.setVisible(false);
    background = new Background();
    background.setVisible(true);        
}

public void windowClosed(WindowEvent e)
{}

public void windowIconified(WindowEvent e)
{}

public void windowDeiconified(WindowEvent e)
{}

public void windowActivated(WindowEvent e)
{}

public void windowDeactivated(WindowEvent e)
{}      
}

this is too much of code. where is Line 13?

Ankit
– Ankit

05/15/2013 05:11:51
Commented May 15, 2013 at 5:11 — Ankit
– Ankit, Commented May 15, 2013 at 5:11

Community · Accepted Answer · 2017-05-23 12:29:22Z

3

Change

public static ArrayList<Contact> contactList;

to

public static ArrayList<Contact> contactList = new ArrayList<>();

in your version contactList is null because you never initialize it and in the write() method you are trying to call size() on it.

Also, there are some serious flaws in your GUI class (in the actionPerformed() method), read this question to fix them: How do I compare strings in Java?

Also remember that textArea.setText(...) will set the complete text for the textArea, so because this is in a loop in your code, the textArea will contain the output of the last iteration of that loop only. In your case it will be only the last contact.

edited May 23, 2017 at 12:29

CommunityBot

11 silver badge

answered May 15, 2013 at 5:12

jlordo

37.9k7 gold badges62 silver badges84 bronze badges

I've updated the code at the top to show the changes I made. At this point it writes the .dat file but only Ԁ appears in the file.

user2380220
– user2380220

05/15/2013 06:11:28
Commented May 15, 2013 at 6:11
@user2380220: You will still only see the last contact in your textarea. Instead of having contactList as a class member, use it as an instance attribute to serialize it.

jlordo
– jlordo

05/15/2013 06:32:50
Commented May 15, 2013 at 6:32
I changed the setText so that it appends. I'm not sure how to go about changing contactList from a class member to an instance attribute/variable. I'm not able to call contactList.read() or contactList.write(), I have to use ContactsCollection.read()/.write().

user2380220
– user2380220

05/15/2013 06:46:23
Commented May 15, 2013 at 6:46
@user2380220: Now, if you have n contacts, you will show every contact n times. You have to get rid of all of the the static keywords and in ContactsCollection and adjust your code to use an instance of it.

jlordo
– jlordo

05/15/2013 06:48:39
Commented May 15, 2013 at 6:48
I removed the static keyword from the ContactsCollection methods and ArrayList instantiation. Now I receive an error stating that non-static methods cannot be referenced from a static context. I thought by including ArrayList<Contact> contactList; as an instance variable in the Contact class I was creating an instance of it?

user2380220
– user2380220

05/15/2013 06:59:54
Commented May 15, 2013 at 6:59

| Show 9 more comments

Collectives™ on Stack Overflow

Java ArrayList null pointer exception

1 Answer 1

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Your Answer

Sign up or log in

Post as a guest

Linked

Related